Acrylic structural adhesives are well-known articles of commerce which are extensively used commercially for bonding metal and plastic materials. The acrylic adhesives typically comprise a mixture of one or more olefinically-unsaturated reactive monomers and curing agents, with cure of polymerization being effected through a free radical or ionic polymerization mechanism. The adhesives preferably contain one or more polymeric materials which may or may not be reactive, that is, capable of being polymerized, per se, or at least capable of interpolymerizing with the reactive monomers, such as grafting onto or crosslinking the growing polymers from the reactive monomer polymerization. In addition, the adhesives can contain other additives for improving adhesion to substrate materials, environmental resistance, impact strength, flexibility, heat resistance, and the like.
The polymerization (cure) of acrylic structural adhesives can be initiated by free radical generators such as peroxygen compounds, usually in combination with accelerators to increase the rate of free radical formation. It has been long recognized, in those applications where at least one substrate was a metal surface, that at least certain metals, such as iron, copper, tin, aluminum, silver and alloys of such metals, exhibited a catalytic effect upon the cure, which was not always positive; that is, in some instances, the metal substrate tended to poison the desired reaction.
There have been many attempts to utilize the catalytic effect of metals. For example, it has been proposed to pretreat non-catalytic surfaces with a catalytic salt solution, such as the resinates of iron, copper or cobalt (see Lees U.S. Pat. No. 3,658,254, column 1, lines 29-52). The Lees patent itself is directed to two-package anaerobic acrylic adhesive compositions whose effectiveness was not influenced by the catalytic or non-catalytic character of the substrate. Skoultchi, in U.S. Pat. Nos. 3,880,956 and 3,957,561, discloses anaerobic acrylic adhesive compositions which are activated by contact with metal surfaces. The compositions of Skoultchi U.S. Pat. No. 3,880,956 are single-package anaerobic compositions containing diazonium salt catalysts which cure through a free radical polymerization mechanism when excluded from air or oxygen and in contact with certain metal surfaces such as iron, copper, tin, aluminum, silver, alloys of these metals, and cadmium, chromium, nickel, and zinc chromate platings. Skoultchi U.S. Pat. No. 3,957,561 discloses one-package anaerobic compositions utilizing a two-component catalyst system comprising at least one diazosulfone compound and o-sulfobenzimide which cure through a free radical polymerization mechanism when the adhesive is excluded from air or oxygen and in contact with active metal surfaces (the same surfaces described in Skoultchi '956). On the other hand, Skoultchi, in U.S. Pat. No. 4,052,244, utilizes copper in the form of a copper salt of saccharin or p-toluenesulfonic acid to provide two-package anaerobic adhesives whose cure is otherwise not dependent on substrate composition. In another development, Skoultchi, in U.S. Pat. No. 4,081,308, discloses two-package adhesives which utilize, in one package, copper saccharinate or saccharin in combination with a soluble copper salt, and in the other package, an alpha-hydroxy sulfone, an alpha-aminosulfone or mixtures of such sulfones, as catalytic agents for the free radical curing of the anaerobic acrylic adhesive compositions. The cure of the Skoultchi U.S. Pat. No. 4,081,308 compositions is independent of substrate composition.
More recent patents (Damico, U.S. Pat. Nos. 4,703,089; 4,855,001; and related 4,857,131) provide one-package acrylic adhesives which cure at ambient temperatures when brought into contact with certain metal surfaces, whether or not air or oxygen is excluded. Those inventions require that the adhesive contain an olefinically unsaturated monomer, a polymeric material, a sulfonyl halide, a transition metal, and an acidic compound. Although these sulfonyl halide-containing adhesives are very effective in many applications, the presence of the halide ions may, in some instances, catalyze or promote corrosion which may lead to the degradation of the adhesive bond. Furthermore, a sulfonyl halide-containing adhesive may be somewhat limited with respect to the inclusion of additional adhesive components, such as polyols, amines, and polyamines, since these hydroxyl- and amine-containing compounds are known to undergo undesirable reactions with sulfonyl halides.
A need therefore exists for a one-package acrylic adhesive which will cure at ambient temperatures when brought into contact with certain metal surfaces regardless of the presence of air or oxygen. It would also be desirable for such an adhesive to eliminate the use of halide-containing compounds that may lead to corrosion and degradation of the adhesive bond and that may prevent the utilization of hydroxyl- and amine-containing compounds, which would otherwise aid in adhesion for certain applications.